The laser printer includes a fixing device for fixing a toner image to a sheet. The fixing device includes a heat roller and a pressure roller in pressure contact therewith. A sheet on which a toner image has been transferred is conveyed to pass between the heat roller and the pressure roller, whereupon the toner image is fixed to the sheet under heat and pressure. The image fixed sheet is then discharged to a sheet discharge portion through a pair of conveyer rollers.
Due to the application of heat to the sheet, the image fixed sheet may be curled in conformance with an outer peripheral surface of the heat roller. The curl may cause sheet jamming or disarray on the sheet discharge portion. In order to avoid sheet curling, a mechanism for changing a sheet nip position by the pair of conveyer rollers has been proposed. More specifically, a lever is pivotally movably provided about an axis of a rotation shaft of one of the conveyer rollers, and the lever supports the other conveyer roller. By the operation of the lever, the nip position between the pair of conveyer rollers can be changed taking a curling state of the sheet into consideration.
Further, in the printer, a first sheet sensor is provided at an upstream side of the fixing device, and a second sheet sensor is provided at a downstream side thereof in order to detect a passing of the sheet. Determination falls “sheet jamming” in case of a non-detection of the sheet at the second sensor within a predetermined time period starting from the detection timing of the sheet at the first sensor.